1. Field of the Invention
The present invention relates to analog/digital mixed mode signal circuitry, and in particular relates to a time-to-digital converter realized by a coupled ring oscillator.
2. Description of the Related Art
A time-to-digital converter (TDC) quantifies time information of a signal event with respect to a reference event. The TDCs have been used in applications in digital phase lock loops (PLLs), physics and laser range finding. The performance of the TDC is characterized by a digital resolution for representing the time information. The TDC is typically implemented by a delay line that comprises a number of delay elements for generating corresponding equally spaced phases. Each delay element is characterized by a propagation delay that limits the digital resolution. Therefore the performance of the TDC depends on accuracy of the propagation delay of each delay element. In practice, the variations of the delay elements due to process variations results in TDC performance degradations.